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A Review of Underground Mine Backfilling Methods with Emphasis On
Cemented Paste Backfill
Morteza Sheshpari Department of Civil Engineering, University of Ottawa, Ottawa, ON,
Canada, K1N6N5, e-mail: email@example.com
ABSTRACT Mining industry is a demanding sector for human advancement. Scarcity of economic minerals near to ground surface has increased deep underground mining. Safety and environmental factors have caused mining companies consider backfilling of mine wastes to avoid mine collapse in further and deeper extraction phases, ground subsidence in mine abandoning and environmental pollution. In this paper main methods in backfilling underground mines with an emphasis on cemented paste backfill method have been reviewed. It was concluded that selection of backfilling method should be performed based on further goals, available equipment and material and economic factors. Also, cemented paste backfill technology is a developing method that can provide better safety factor for underground mining environments and suitable preventive method for environment pollution by disposing toxic waste minerals underground.
BACKGROUND ON THE DIFFERENT TYPES OF BACKFILL Underground mining of economic minerals creates voids in different shapes including stope, cave,
room, goaf, and gob void forms. These underground voids create instability hazards for extraction of adjacent pillars that contain economic minerals, or subsidence hazard for infrastructures on the ground during operation, or after mines are abandoned. Therefore during historical development of mining technology, several methods of refilling or backfilling for those voids have been developed. Some of the common methods of backfilling which are used according to economic factors and further goals such as development of mine or abandoning the mine are rock backfill, hydraulic backfill, cemented paste backfill, and silica alumina-based backfill methods.
Rock Backfill Method When extraction of economic minerals considers deeper ground, number and volume of voids
including stopes, goaf, and goab increases conventionally, and waste rocks are directly dumped on the ground surface mining fields (Castro-Gomes, 2012, Wang et al. 2013). Disposal of waste rocks has been a challenge due to their large volumes and containment of contaminative metal elements. Winds can spread dust and micro particles, and rain can create leachate from heavy and toxic elements stored in dumped rocks and pollute the hydrologic and hydrogeologic environments (Smuda et al. 2007; Poisson et al. 2009; Wang et al. 2013). The voids created from mining can induce instability and settlement hazards. Collapse of the created voids including stopes, goaf, and etc. is one of main factors in rock burst and potential land subsidence (Jirnkov, 2007; Helm et al. 2013; Wang et al. 2013).
Vol. 20 , Bund. 13 5184 Rock backfill can be described as a technology for transportation of backfill forming components such as stone, gravel, soil, industrial solid waste using manpower, gravity or machinery equipment to fill underground mined voids and production of compressed backfill body. Backfill material are usually produced from waste rocks by crushing, sieving and mixing by machinery equipment by taking the particle size distribution pattern into account (Yao et al. 2012). Based on equipment that are used for transfer of filling materials, backfill scheme can be divided into three Schemes. (I) side-dump tramcar, (II) belt conveyor and (III) combination of truck and scrapper, Table 1 (Wang et al. 2013)
Table 1: Advantages and disadvantages of rock backfill systems (Wang et al. 2013)
Scheme Applicable areas Advantages Disadvantages I gobs small in size, (a)easy for operation; (a)high labor intensity; with good stabilities (b)few devices input (b)low efficiency
II filling workface (a)high capacity; (a)expensive devices; nearby hanging wall (b)flexible operation; (b)complex structure (c)high gob utilization
III Mines with truck (a)high capacity; (a)critical tire wear; Transportation (b)wide application range ; (b)gas pollution
One of the cases that rock backfill is selected as a feasible and economic way and applied to treat excavated gobs underground is White Bull mine in China. Locomotive traction and tramcar haulage were used for waste rock transportation (Wang et al. 2013).
Figure 1: An example of rock backfill used for underground goaf refilling in China, one of
the goafs that intersects with level drift is shown above (Wang et al. 2013).
Rock backfill method is an economical back filling method which is feasible and applicable in some but not all cases in underground mining. This method decreases waste material on the surface and expands usable land on the ground, decreases environmental pollution by transferring waste rocks to deeper levels out of rain contact, increases stability of mined area and decreases land subsidence and rock bursts due to stress pattern changes.
Hydraulic backfill Method Hydraulic backfill is one of the refilling technologies that use water as the transportation medium
to convey the hydraulic backfill materials, such as waste tailings, water hydrophilic slag, mountain
Vol. 20 , Bund. 13 5185 sand, river sand, and crushing sand to fill underground mined voids like stopes (Yao et al. 2012). An example of hydraulic backfill is shown in Figure 2. The access drives to mine are blocked with bulkheads and a drainage system is inserted into the bulkhead so water from backfill drains out of the stope. Hydraulic backfill material is poured inside the stope from fill hole in the crown of stope. While the height of hydraulic backfill material is increased, free water is accumulated on top of the backfill. All water should be drained through bulkheads or as a seepage water or as decanted water (Potvin et al, 2005). Hydraulic backfill has following properties, maximum particle size is less than 1m and very fine particles are removed generally to create permeability in the backfill material. Therefore particles with a size less than or equal to10 m should be less than 10 percent and in most cases much lower than that in total (Potvin et al, 2005). Slurries for hydraulic backfill have densities between 40-50% (Solid parts volume). Permeability of hydraulic fill material should be between 10-5 to 10-6 m/s. Excess water in backfill material is drained by gravity, and assisted by drainage out of the backfill. Porosity of emplaced hydraulic backfill is 50%, but porosity of 30% has been reported. Hydraulic backfill can be placed cemented or non-cemented, which the later one is one of the cheapest methods in mine when small size waste particles are available (Potvin et al, 2005). Strict rules should be applied in design of backfill and barricades, and controlling of backfill material properties. Increasingly cemented paste backfill is being replaced for hydraulic backfill where strength is required from backfill or waste material contain higher amount of very fine particles (Potvin et al, 2005). Figure 3, shows grain size distribution of 20 hydraulic backfill materials from Australian mines along with cemented hydraulic backfill, and paste backfill materials grain size distribution. It can be seen that hydraulic backfill material fall in narrow band along with cemented hydraulic backfill material. Effect of cement on the grain size distribution is limited. In the paste backfills generally fine fraction is larger than hydraulic backfills or cemented hydraulic backfills, but their colloidal fraction (finer than 2 micrometer) is negligible.
Figure 2: Examples of hydraulic backfill in open stope (Potvin et al, 2005; Sivakugan et al,
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Hydraulic backfill grain distribution usually contains Sandy silt, Silty sand (SM-ML). The clay fraction is removed by a process named desliming, in which entire backfill material pass through hydrocyclones by circulation and the clay fraction is removed and then deposited into the tailings dam. Hydrocycloned silty sandy backfill material is transferred in the form of slurry by pipelines to underground voids (Sivakugan et al, 2006). However, more solid material reduces water content in drained status in the hydraulic backfill, but creates problem in smooth transfer through pipes. In current practice 75-80 % of content for solid particles is common but considering 75% solid particle with specific gravity of 3, almost 50% of slurries volume would be water which requires drainage facilities by using special porous bricks in building barricades in front of drawpoints or horizontal drives (Sivakugan et al, 2006). Drainage is the most important factor in designing hydraulic backfills, because negligence in this section has caused several deadly incidents due to liquefaction, rush in, and piping problems, and not using porous barricades (Bloss and Chen 1998; Tolarch, 2000). Threshold for permeability of hydraulic backfill material should be more than 100 mm/h and higher values accelerate drainage of backfilled stope (Herget and De Korompay, 1978).
Figure 3: Grain size distribution of hydraulic backfill (from 20 mines in Australia) and
cemented hydraulic and paste backfills (Sivakugan et al, 2006). Laboratory tests and field monitoring by other researchers showed that threshold suggested by
Herget and De Korompay (1978), is conservative. In other words permeability in the range 7-35 mm/h by Sivakugan et al., (2006) in controlled laboratory environment produced satisfactorily results in the stopes due to much higher values that happen in the in-situ conditions. Kuganathan (2001) and Brady and Brown (2002), suggested that permeability values between 30-50 mm/h are significantly l